From a distance the island looked uninhabitable: a mangrove shore and thick vegetation, surely a haven for sly crocodiles and giant mosquitoes. But drawing closer I saw a tiny jetty and a few tankwas, supple papyrus boats that flex like eels on the waters of Lake Tana.

We landed and headed up some stone stairs. The small island now seemed immense, its jungle dense and cool with pigeons cooing hypnotically in the shade. Gaining the hill we passed a low gateway and emerged into a clearing. Before us, like an outsized tribal hut, stood the monastery of Kibran Gebriel. Fishing nets curtained its vaguely Romanesque arcade, to thwart nesting birds. An ancient lithophone hung by the perimeter wall, its peculiar timbre rarely heard now, even here. Our guide sought a priest.

The Ethiopian Orthodox Church is among the world's oldest Christian denominations. One of its spiritual nodes was here on the shores and islands of Lake Tana, Ethiopia's largest lake and host to several shifting capitals. Most of Tana's churches and monasteries were founded in the 14th and 15th centuries and rebuilt two or three hundred years later. We had come to admire their location, atmosphere and treasures. The priest appeared with his keys and the heavy wooden doors swung open. Reed mats and a few rugs covered the floor, while porphyry pillars soared into the gloom. But it was the paintings - immense, colourful depictions of saints and sinners, cherubs and demons - which commanded our attention. Most were painted on cloth later glued to the outer walls of the holy of holies.

Kibran Gebriel is among the most celebrated of Tana's twenty or so churches, mainly for its collection of books and manuscripts. In a nearby watchtower, now a tiny repository, our priest-turned-curator proudly held up ceremonial crosses and cradled delicate triptychs, before producing ancient crowns and beautiful 16th-century illustrated gospels. These churches seem to thrive on legends and relics. Tana Chirkos island monastery was reputedly home to the Ark of the Covenant for 800 years and has a rock said to bear Christ's footprint. Daga Istafanos, a more remote island with the largest monastery, has a mausoleum with the mummified remains of five Ethiopian emperors.

We set off down the hill and back to our skiff. On we chugged towards the lush Zege Peninsula, with fleets of pelicans on the horizon and cormorants gliding just feet from our prow to dive for fish. Zege boasts several churches, the most famous being Ura Kidhane Mihret. Like Kibran Gebriel it stands in a walled clearing, but is far more imposing.

It's with good reason the old grass roof is being replaced with corrugated sheets. These 500-year-old murals are the most extensive and vibrant of all Tana's churches, and the authorities are determined not to let rain spoil them. For many this rich montage of biblical scenes fused with medieval Ethiopia is a place of pilgrimage. On the shores of Lake Tana, the old world and the new seem but a brushstroke apart.

Lake Tana Factfile

Amar Grover visited Ethiopia with Tim Best Travel (www.timbesttravel.com) and British Airways (www.britishairways.com) who fly three times a week from London Heathrow to Addis Ababa.

Bahar Dar is the most convenient base for visiting the churches. You will need at least a good half-day to visit Kibran Gebriel and Ura Kidhane Mihret by boat; more if you want to visit other churches and explore the Zege Peninsula.

Visiting both Tana Chirkos and Daga Istafanos requires another full day and you might wish to combine this with Narga Selassie monastery on Dek Island. Boat charges range from about 250 birr (US$30) to 1000 birr if arranged locally.

All churches charge entrance fees, typically around 15-20 birr (about US$2-2.50), sometimes up to 50 birr. Note that women are not allowed inside many monasteries, including Daga Istafanos, Tana Cherkos & Kibran Gebriel. Modest dress is recommended for men and women.